1. The Field of the Invention
This application relates to fiber optic transmitters, and, more particularly to wave division multiplexed (WDM) transmitters.
2. The Relevant Technology
One means for increasing the capacity of a fiber optic network is to use wave division multiplexing (WDM) wherein optical signals having different wavelengths are transmitted simultaneously through the fiber. For example, the industry standard for the transmitter optical sub-assembly (TOSA) for 100 Gb/s Ethernet is expected to include wavelength division multiplexing of 4 transmitters at 25 Gb/s.
One proposed transmitter implementing this standard includes a co-package of an array of externally modulated lasers (EML), such as a monolithically integrated DFB laser and external electro-absorption modulator, a wave division multiplexer (WDM), an array of photodiodes, and output fiber coupling assembly. Other proposed transmitters include a directly modulated arrayed distributed feedback (DFB) laser.
Externally modulated lasers have the significant disadvantage of being bulky due to the requirement of both a laser and an external modulator. They further require more power inasmuch as both the laser and external modulator are powered. They also require a high degree of integration, which makes them difficult to manufacture with high yields. EML have been demonstrated at 40 Gb/s with extinction ratio>=10 dB and dispersion tolerance of ˜40 ps/nm (Okayasu, M. et al, A 1550-nm 40-Gbit/s electro-absorption DFB laser diode module for transponders with very short reach (<2 km) applications, LEOS 2004, page: 513-514).
Directly modulated multi-wavelength integrated DFB laser arrays have been deployed widely as wavelength selective laser source, but the extinction ratio (ER) of most directly modulated lasers is less than 7 dB, which is less than the required ER for most applications. The highest ER reported for 40 Gb/s direct modulated DFB laser is 5 dB, Nakahara K., et al, “40-Gb/s direct modulation with high extinction ratio operation of 1.3-um InGaAlAs multiquantum well ridge waveguide distributed feedback lasers,” IEEE Photonics Technology Letters, Vol. 19, October, 2007, pp. 1436-1438. Other directly modulated lasers provide better extinction ratios, however WDM system incorporating such lasers have not been demonstrated.
In view of the foregoing it would be an advancement in the art to provide a compact WDM transmitter using a directly modulated laser array that further provides an extinction ratio at least as high as 7 dB. It would be a further advancement in the art to provide such a transmitter that was readily manufacturable.